Form Issue: Cannot Submit

Hi There,

I am having an issue with Contact Form 7 on my website.
It is not allowing users to submit a form.
It throws an error “There was an error trying to send your message. Please try again later.” and it is encapsulated in a red box.

Here is some background:

My domain is hosted by one company and the server is hosted by a second.

I originally built a site and stuffed something up and decided to start from scratch.
I followed this video to restore my HostGator server to start afresh.

I reinstalled WordPress, I installed the Uncode theme, all the plugins, including Contact Form 7 and a few others that I wanted to try out. I decided upon Contact Form 7.

I have placed it on my site but after the new install, it no longer submits.

Previous to the new install, when a user submitted a form, it would be emailed to my outlook email. I believe it did this via PHP.

What I have tried:

I tried installing WP-MAIL-SMTP and using that tool to see if the email functionality works.

When I try PHP, it throws the error:

bool(false)

Could not instantiate mail function.

When I try SMTP, it throws the error:
My email is hosted on Office365 and I use the Office365 SMTP Host details along with my credentials. It communicates to Office365 host but my details fail authentication.

I have spoken to my domain provider and they said that it isn’t an issue on their end.
I tried speaking to HostGator on live chat but they seem to think it isn’t their issue either.

Can anyone help?

Thank you!

The page I need help with: [log in to see the link]